The long-term goal of this research program is to gain knowledge about mechanisms which determine the radiosensitivity of mammalian cells, particularly those involved in the response of stationary or very slowly proliferating cells to X-irradiation. We will utilize three different types of cells: 10T1/2 mouse embryo fibroblasts, human diploid cells and human tumor cells. Four cellular endpoints will be studied in parallel: survival, mutagenesis, and the induction of chromosome aberrations and sister chromatic exchanges (SCE). We will investigate the relationships between these cellular effects, and try to correlate them with molecular DNA repair processes. Techniques for measuring DNA repair will include host cell reactivation of irradiated herpes virus, alkaline elution assay for DNA strand breaks and crosslinking, and measurement of endonuclease sensitive sites. Cellular and molecular phenomena will be studied during the repair of potentially lethal damage (PLD) in stationary phase 10T1/2 and human diploid cells treated with X-rays, UV light or mitomycin-C. The frequencies of induced mutations, SCE and chromosome aberrations will be studied in parallel experiments, and correlated with molecular repair. In particular, evidence will be sought which implicates the action of error-prone repair processes in the cellular effects induced by these agents. Further information on mechanisms will be obtained through the study of skin fibroblasts isolated from individuals who are unusually susceptible to the development of cancer, especially radiation induced cancer, either because of family history or because they have a recognized genetic syndrome associated with an increased frequency of spontaneous and/or induced cancer. We will measure the sensitivity of these cells to the cytotoxic and mutagenic effects of radiation in vitro (mutations to 6-Thioguanine resistance) during PLD recovery, and seek evidence for molecular DNA repair defects. Finally, rapid split dose recovery and PLD repair will be studied in plateau phase human diploid and human tumor cell lines, and a relationship between processes and tumor radiocurability in vivo sought.